Methods

The methods used to perform photometry depend on the wavelength regime under study. At its most basic, photometry is conducted by gathering radiation in a telescope, perhaps passing it through specialized optical filters, and then capturing and recording the light energy with a photosensitive instrument. The set of passbands (filters) is called a photometric system.

Historically, photometry in the near-infrared through long-wavelength ultra-violet was done with a photoelectric photometer, an instrument that measured the light intensity of a single object by directing its light onto a photosensitive cell. These have largely been replaced with CCD cameras that can simultaneously image multiple objects, although photoelectric photometers are still used in special situations, such as where high time resolution is required.

CCD photometry

When using a CCD camera to conduct photometry, there are a number of possible ways to extract a photometric measurement (e.g. the magnitude of a star) from the raw CCD image. The observed signal from an object will typically be smeared (convolved) over many picture elements or pixels according to the point spread function of the system. This broadening is due to the optics in the camera telescope as well as to astronomical seeing (twinkling). When obtaining photometry for a point source (an object with an angular diameter that is much smaller than the angular resolution of the telescope), the goal is to add up all the light from the object and subtract the light due to the sky. The simplest technique, adding up the pixel counts within a circle centered on the object and subtracting off the product of the per-pixel average nearby sky count and the number of pixels within the circle, is known as (synthetic) aperture photometry.[2] When doing photometry in a very crowded field, such as a globular cluster, where the profiles of stars overlap significantly, one must use de-blending techniques, such as point spread function (PSF) fitting,[3] to determine the individual fluxes of the overlapping sources.

Computer software packages are available for aperture photometry and PSF-fitting photometry, in some cases at no cost. Aperture Photometry Tool is an example of a recently developed application,[4] which has a graphical user interface and can be downloaded free of charge via aperturephotometry.org.

Calibrations

After determining the flux of an object in counts, the flux is normally then converted into instrumental magnitude. Next, one must calibrate the measurement in some way. Which calibrations are needed depend in part on what type of photometry is being done. One typically speaks of performing differential, relative or absolute photometry. Differential photometry is the measurement of changes in the brightness of an object over time; these measurements are compiled into a light curve of the object. Relative photometry is the measurement of the apparent brightnesses of multiple objects relative to each other. Absolute photometry is the measurement of the apparent brightness of an object on a standard photometric system; these measurements can be compared with other absolute photometric measurements obtained with different telescopes or instruments. In most cases, differential photometry can be done with the highest precision, while absolute photometry is the most difficult to do with high precision. In general, accurate photometry is more difficult when the apparent brightness of the object is fainter.

To perform differential photometry, one must correct measurements for temporal changes in the sensitivity of the instrument as well as changes in the atmospheric extinction through which the object is observed (when observing from the ground). This is typically done by simultaneously observing a number of comparison stars, which are assumed to be constant, together with the object(s) of interest.

To perform relative photometry, one must correct measurements for spatial variations in the sensitivity of the instrument and the atmospheric extinction. This is often in addition to correcting for their temporal variations, particularly when the objects being compared are too far apart on the sky to be observed simultaneously.

To perform absolute photometry one must correct for differences between the effective passband through which an object is observed and the passband used to define the standard photometric system. This is often in addition to all of the other corrections discussed above. Typically this correction is done by observing the object(s) of interest through multiple filters and also observing a number of photometric standard stars. If the standard stars cannot be observed simultaneously with the target(s), this correction must be done under photometric conditions, when the sky is cloudless and the extinction is a simple function of the airmass.

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